Recycle catalytic conversion of liquid reactants



Jan. 17, 1961 E. v. BERGSTROM RECYCLE CATALYTIC CONVERSION OF LIQUID REACTANTS Filed Sept. 22, i958 2 Sheets-Sheet l INVENTOR.

ATTORNEY Jun. 17, 1961 E. v. BERGSTROM 2,963,610

RECYCLE CATALYTIC CONVERSION OF LIQUID REACTANTS Filed Sept. 22, 1958 2 Sheets-Sheet 2 F47? K J 56 V gl ENTOR. BYE/H3O i '3- Al ATTORNEY 2,968,610 Patented Jan. 17, 1961 RECYCLE CATALYTIC CONVERSION OF LIQUID REACTANTS Eric V. Bergstrom Byram Conn., assignor to Socony Mobil Oil Company, Inc., a corporation of New York Filed Sept. 22, 1958, Ser. No. 762,591

,5 Claims. (Cl. 208100) This ipvention relates to the catalytic conversion of high boiling liquid reactants to lower boiling gaseous products. More particularly, it relates to such conversion processes in which the liquid material remaining after the conversion is continuously recycled to the conversion zone. t

Typical of the processes to which this lnventlon may be applied are the catalytic cracking and h ydrocracking of high boiling hydrocarbons to lower boiling products.

In conventional operations of catalytic hydrocarbon conversion processes, it is rare that all of the fresh charge stock is converted to the desirable gaseous products. Usually there remains some material of a boiling range substantially higher than that of the desired products. It is therefore customary to continuously recycle this heavier material to the conversion zone.

This recycle operation has other advantagesbesides effecting ultimate conversion of all of the charge stock to desired products. Where the conversion is exothermic, such as for example, the hydrocracking of hydrocarbons, the recycle acts as a heat absorbing medium which helps to prevent a temperature runaway in the reactlon zone. In addition, it has been found that a large volume of recycle in such processes as hydrocracking acts to retard deposition of carbonaceous contaminants on the catalyst, and, therefore, prolongs the on stream time between catalyst regenerations.

One of the major drawbacks involved with a recycle operation, however, has been the high cost of the equipment, particularly pumps, needed to effect the recirculation of the large liquid volume required. This is particularly true of high pressure operations and, in addition, the maintenance of high pressure pumps is very difiicult.

There has now been developed a catalytic conversion operation employing recycle in which no pumps are required to recirculate the recycle stream.

A major object of this invention is to provide an economical and efiicient method and apparatus for the catalytic conversion of liquid reactants to other products in which higher boiling portions of the conversion zone efiluent are continuously recycled to the conversion zone.

Another object of this invention is to provide a continuous recycle system for the conversion of high boiling liquid hydrocarbons to lower boiling gaseous products without the use of outside force to elfect the recycle.

A specific object of this invention is to provide a process and apparatus for the hydrocracking of high boiling hydrocarbons in which there is continuous recycle to the reactor of material heavier than the desired products.

Broadly, in this invention, a liquid reactant is passed to a catalytic conversion zone wherein a portion of the reactant is converted to gaseous products while some material remains-in the liquid state. The mixture of liquid and gaseous materials so produced is flowed upwardly as a confined stream to a separation zone above the conversion zone. In the separation gaseous products are removed from the liquid which accumulates in the lower section of the separation zone. This liquid is then flowed downwardly as a confined stream to the con- .version zone and it is'merged with fresh reactant prior to entry into the conversion zone. The degree of conversion is controlled so that a suificient quantity of gaseous material is at all times formed by the conversion to cause the stream flowing upwardly from the conversion zone to the separation zone to have a hydrostatic pressure at the conversion zone outlet less than the hydrostatic pressure exerted at the conversion zone inlet by the inlet recycle stream. When this occurs the liquid will be recycled continuously and the product will flow from the conversion zone continuously without employment of pumps.

As used in this invention, the terms gaseous and liquid relate to the state of the material in question under the temperature and pressure conditions at which it exists at the point of reference without any regard to what its state may be under standard conditions.

This invention will be better understood by referring to the attached drawings, of which Figure l is an elevational view, partially in section, of one form of apparatus 'in which the operation of this invention may be conducted;

Figure 2 is an elevational sectional view of a second apparatus capable of operation according to this invention; and

Figure 3 is an elevational sectional view of an apparatus employing a preferred form of this invention.

These drawings are diagrammatic in form. V

This invention will be explained in connection with one type of reaction, the hydrocracking of heavy hydrocarbons to produce gasoline and lighter products. The invention will, however, be seen to obviously apply to any other catalytic conversion reactions, including the conventional catalytic cracking of hydrocarbons.

Turning now to Figure 1, there is shown a reaction zone 10 in which there is centrally disposed a catalyst bed 11. Any of the conventional hydrocracking catalysts may be used. These catalysts normally consist of a cracking component and a hydrogenation component. As non-limiting examples the hydrogenation component may be nickel, molybdenum, platinum, palladium, ruthenium, tungsten or cobalt or the oxides or sulfides of these materials. The cracking component might be a composite of silica and alumina, silica and zirconia or silica and magnesia. One suitable catalyst consists of 0.05 to 20 percent by weight of platinum deposited on silica-alumina.

The operating conditions within reaction zone 10 may vary widely, depending on the catalyst used. Pressures from a few hundred pounds per square inch to several hundred atmospheres have been suggested. Temperatures in the range 400 F. to 950 F. have been disclosed.

Catalyst bed 11 is supported on both sides by perfo rated surfaces 12 and 13. These surfaces may, if desired, be screens. There is provided between the wall of zone 10 and perforated, surface 12 a space for hydrocarbon flow 14 and a similar space 15 is provided below perforated member 13. Separator 16 is situated above reaction zone 10. The lower section of the separator is connected to the inlet end of reactor 10 by means of passageway 17, while the upper portion of the separator .connects to the outlet end of reactor 10 by passageway In the operation of the apparatus of Figure 1, a suitable high boiling charge stock enters the system through passageway 19. This charge stock may be, for example, a very heavy residual material, a topped crude oil or a heavy gas oil. A sufficient quantity of hydrogen accompanies this charge stock for use in the hydrocracking 'drocarbons are converted to gaseous products.

, reactor 30 through passageway 36. i" accompanied by any desired quantity of'hydrogen. In

' gaseous hydrocarbons.

ratio might be in the range 280. The fresh charge merges with liquid recycle flowing downwardly from separator 16 in passageway 17. The mixture passes into the inlet end of reactor through space 14 and through catalyst bed 11. Within the reactor a part of the hy- These gaseou products are dispersed through the remaining liquid and the entire product stream flows upwardly through passageway 18 into separator 16. Within the separator the gas disengages from the liquid and is removed through passage 20 while the liquid forms an accumulation 21 in the bottom of the separator.

In this invention the flow of product stream to the separator and the return of liquid recycle from the separator are accomplished without the use of mechanical pumping. This is done by controlling the degree of conversion within the reaction zone 10 to produce sufficient quantity of gaseous products that the hydrostatic head at the reaction zone outlet 22 is less than the hydrostatic head at inlet 23. The more gaseous products that are produced, of course, the lower will be the over-all density of the fluid within passage 18 and the lower will be the hydrostatic head exerted by it at point 22. Thus, the circulation can be slowed down or speeded up by controlling the degree of conversion and any desired recycle ratio attained. Recycle ratio is the volumetric ratio of recycle liquid to fresh charge supplied to the conversion zone.

In order to, at all times, maintain suflicient liquid recycle available and to prevent the separator from filling with recycle, it is preferred that a level control device 24 be utilized to measure the surface of the liquid accumulation 21 and actuate valve 25 on withdrawal line 26 to remove some of the liquid recycle from the separator should its quantity become inordinately large and to stop removal when the level of accumulation 21 drops too low.

Turning now to Figure 2, there is shown another form of apparatus which is a part of this invention. The reactor shell 30 is an upright cylindrical vessel. Catalyst bed 31 is maintained within vessel 30 and is annular in shape. An annular inlet space 32 is provided on one side of catalyst bed 31 and an annular outlet space 33 on the other. These spaces are maintained by inner perforated member 42 and outer perforated member 43,

both concentric with passage 35. Forming the inside surface of inlet space 32 is a cylindrical conduit 35, to the upper end of which is attached a somewhat larger cylindrical member 34 to form a centrally disposed funnel-shaped member with upper and lower ends spaced away from the ends of vessel 30. Solid member 40 closes off and confines the upper end of catalyst bed 31 and the annular space containing the bed, and also closes off the upper end of annular space 32.' Similarly, solid member 41 confines the lower end of bed 31 and closes off the lower end of annular space 33.

Inoperation, the fresh charge enters the bottom of This charge may be space 37, which is below catalyst bed 31, the fresh charge mixes with liquid recycle which descends through passageway 35. The mixture is forced upwardly into inlet space 32, then laterally through catalyst bed 31, in which the desired reaction takes place to produce a quantity of The product stream, a mixture of gas and liquid, flows upwardly and spills over the top of funnel-shaped member 34. Within member 34 the gas disengages from the liquid and is removed from the reactor through passageway 38, while liquid flows downwardly through passageway 35. Here, again, in this operation, the reaction is controlled to produce a sufficient quantity of gaseous material that the hydrostatic head of the fluid product stream flowing from the reaction bed is less than the hydrostatic head of the liquid stream reaction. Typically, the hydrogen to hydrocarbon molar feeding the bed. For-simplicity it is possible to determine the hydrostatic driving force in this particular species of this invention by measuring the head of fluid from the liquid surface level within funnel-shaped member 34 to any level below the upper endof catalyst bed 31.

Thus, in the drawing, the hydrostatic head H of liquid recycle should at all times be greater than the hydrostatic head H: of mixed liquid vapor product, and the greater the difference between the two the greater the driving force and the faster the liquid will recirculate.

In order to achieve uniform distribution of charge across the annular bed, the perforations 39 on the inlet wall confining the bed should be progressively larger and/or progressively closer together as they. are further removed from the bottom of the catalyst bed. Stated another way, the percentage of the area of the upright cylindrical member 42 taken up by perforations 39 progressively increases at progressively higher levels.

A more preferred form of this invention is disclosed in Figure 3. It is considered that, in a commercial size arrangement, the hydrocracking uni-t might consist of a multiplicity of units like that shown in Figure 3, each capable of independent operation. However, within the broad scope of this invention a single such unit may be used.

The hydrocracking reactor shown in Figure 3 consists of a spherical shell 50 with internal insulation 51. Horizontal, forarninous partitions or screens 52 and 53 are provided across the upper and lower ends of reactor 50. To assist the liquid'distribution, a narrow layer of inert particles 54 is provided on top of partition 53. Above the inert particles and below partition 52 is the bed of hydrocracking catalyst 65, all within a liner 55.

The hydrocarbon charge and hydrogen are introduced to the system through conduit 56. These materials mix with recycle hydrocarbons from conduit 57 and enter the lower end of reactor 50. Reactor effluent exits by means of conduit 58 and flows upwardly into the higher section of an inclined condenser 59. In condenser 59 the material which it is desired to recycle that did not remain in liquid phase is condensed. The cooling fluid is introduced through conduit 60, flows through tubes 61 and'is removed through conduit 62. While any coolant may be used, it is preferred that fresh cold hydrocarbon charge be employed so that the heat extractedis not lost to the system. Gaseous product and hydrogen leave condenser 59 via conduit 64 while liquid collects adjacent the lower end of the condenser 59 behind a weir 66. It is to be noted that the use of weir 66 makes it unnecessary to employ the float and valve device of the previously discussed examples of this invention. It the liquid builds up in accumulation 63, it will simply spill over weir 66 and exit with the vapor product through con-' duit 64. The liquid recycle returns through conduit 57. A valve 70 may be employed in conduit 57 to control the flow of recycle liquid through that conduit.

As in all species of this invention, the rate of reaction in bed 65 is controlled so that the density of the product stream from the reactor is sufliciently less than the density of the recycle stream to achieve the desired recycle rate without the use of pumps. I

One advantage of the system shown in Figure 3 is its employment of a spherically shaped reactor rather than the conventional cylindrical reactor. Spherical reactors have a substantially lower reactor pressure drop than conventional upright cylindrical reactors.

Another advantage of the spherical reactor is its high tolerance to accumulation of scale. In hydrocracking, hydrogen sulfide will always be present, in much of the system, at high temperatures. This results in a rather high rate of corrosion of the metal parts in those portions of the system even though corrosion-resistant alloys are employed. The scale so formed regularly breaks off and is carried into the reaction bed and plugs the pores between catalyst particles, thereby causing a sharp rise in the pressure drop through the bed. It has been found that a vertical cylindrical reactor suffers a much more pronounced pressure drop build-up due to scale than the spherical reactor.

It will be readily apparent that this invention will have wide application to a variety of catalytic conversion processes in which liquid is converted to gaseous products. Of course, the invention will be most advantageous where large volumes of recycle must be handled and thus will find particular application in such processes as the hydrocracking of heavy residual materials where the con- 1 version on a once through basis is quite low. As noted above, it will also be advantageous in hydrocracking because the large recycle ratio can be used to absorb heat and because high recycle ratios in this process tend to promote catalyst life.

As above explained, the hydrostatic head at the reaction zone inlet must be greater than the hydrostatic head at the reaction zone outlet. Just how much this difference should be will depend on the particular conversion process to which the invention is applied, the size and depth of any catalyst bed used, the volume of recycle that must be handled and the resistance to fluid flow of the circulating system. In general, it has been found for hydrocracking of residual materials in the general manner above described, that the hydrostatic pressure at the reaction zone inlet should be from 3 p.s.i. to 15 p.s.i.

greater than the hydrostatic head of the fluid stream at t the reaction zone outlet.

Stated another way, the hydrostatic pressure exerted by the recycle stream at the point where it mixes with fresh hydrocarbon charge must be sufiicient to force the recycle stream into the mixture at the desired rate, force it through the reaction bed of catalyst, and force the effluent from the conversion zone upwardly into the cooling or condensing zone. The major item of pressure gradient which must be overcome will be that required to force the conversion zone effiuent upwardly into the cooling zone. The technique of this invention is to adjust the density of the conversion zone efliuent by adjusting the degree of conversion to gaseous products in the reaction zone to achieve the desired recycle ratio. Obviously, the density of and hydrostatic pressure exerted by the recycle stream will not change much over the course of time as the unit is operated. Such changes as occur will normally be slight variations due to changes in the temperatures in the stream. Thus, as the density of the conversion zone efiluent is decreased there is less force exerted against the recycle stream and its flow rate will increase. Conversely, as the density of the reactor effluent is increased the flow rate of recycle must decrease.

When this invention is used in hydrocracking the hydrogen is preferably added adjacent the inlet end of the reaction bed. since this gas will then assist in maintaining the difference in hydrostatic head between product and recycle streams which is required to practice this invention.

If desired, in less preferred forms of this invention the fresh charge may be supplied through a jet or eductor which can act to assist in procuring the necessary recycle rate.

Another feature of this invention is that the recycle may be returned to the reactor at a temperature not too materially different from the reaction temperature. There is no need to completely cool the recycle and then reheat it to reaction temperature as there is in conventional systems. The absence of recycle in the heater materially affects the corrosion problem in the heater tubes and makes it possible to employ a less expensive alloy in these tubes.

Example I In one design of an apparatus according to this invention along the lines of Figure 2, the reactor had an inside diameter of 65 inches and was 94 feet high. The height of the upper section of the inner funnel-shaped member,

34 was 8 feet and it was located with its upper end 3 feet from the upper end of the reactor. The lower section of the funnel-shaped member 35 was '82 feet 1n length and its lower end was 1 foot from the bottom of the reactor shell. The outside diameter of member 34 was 56 inches, and the outside diameter of member 35 was 24 inches. Perforated surface 42 had a diameter of 36 inches and perforated surface 43 had a diameter of 56 inches. Both surfaces had their lower ends 3 feet from the bottom of the reactor shell and were feet high. The fresh charge to this unit might be 4,000 barrels per day of 8 API refinery residuum. The recycle rate might be 20,000 to 80,000 barrels per day. The catalyst to be used would be conventional cobalt molybdate on alumina of a size within the range 15 to 30 mesh by Tyler Standard Screen Analysis. The re.- action zone would operate at 1,500 to 3,000 pounds per square inch and a temperature of 800 F. Fresh charge and hydrogen would be supplied at 400 F. The volume of gaseous product, including hydrogen to be recycled, woud be about 2,000 to 5,000 standard cubic feet per barrel. The catalyst bed was estimated to have a pressure drop of 4.0 pounds per square inch. The liquid recycle stream would have a density of 20 to 50 pounds per cubic foot and the gaseous product-recycle stream a density of 8 to 20 pounds per cubic foot.

Example II In a hydrocracking unit designed along the lines of the unit shown in Figure 3, reactor 50 was 12 feet, 2% inches in diameter with a wall thickness of 5 inches. The catalyst bed was 8 feet, 2 inches across at its narrowest level and 10 feet, 11%inches across at its widest level. The catalyst bed was 7 feet, 6 inches high. Condenser 59 was inclined at an angle of 15 degrees and Was designed to provide six passes for the material in the tubes and one pass for the material in the shell. Weir 66 was about 2 feet high. Tubes 61 were 1 inch outside diameter and 16 feet in length. Conduits 57 and 58 were 8% inches outside diameter with a inch wall thickness.

This unit was designed to hydrocrack 4,410 barrels per day of hydrocarbon charge. This charge and 11,011 pounds per hour of a hydrogen-containing recycle gas having an average molecular weight of 7.1 would be admitted through conduit 56 at 915 F. and 2,000 pounds per square inch gauge. The charge and gas might be joined by a volume of recycle equal to the volume of charge at about 750 F. and the mixture would pass into the reactor at about 850 F. The reactor efiiuent might be at about 930 F. and 1997.4 pounds per square inch. In the condenser this product would be cooled to 750 F. The fresh charge would be employed as a coolant in the condenser which would raise the temperature of the charge from F. to 650 F. The average density of the recycle stream in conduit 57 would be about 30 pounds per cubic foot and the reactor effiuent in conduit 58 would have an average density of 8 pounds per cubic foot.

It is intended to cover herein all modifications of the examples of this 'nvention, chosen for purposes of disclosure, which fall within the spirit and scope of the invention.

I claim:

1. An apparatus for the catalytic conversion of liquid hydrocarbons, which comprises in combination: an upright enclosed cylindrical reaction vessel; an upright funnel-shaped member concentrically within said reaction vessel, the lower end of the spout of said funnelshaped member being spaced away from the bottom of said vessel and the upper end of said funnel-shaped member being spaced away from the upper end of said vessel; a first cylindrical perforated member of, greater diameter than the spout of said funnel-shaped member concentrically about said spout so as to define a first annular space between said perforated member and said spout; a second perforated member of larger diameter than said first perforated member and smaller diameter than said vessel concentrically about said first perforated member so as to define a second annular space between said two perforated members and a third annular space between said second perforated member and the inner wall of said vessel; means closing off the upper end of said first annular space; means closing off both ends of said second annular space; a bed of solid cracking catalyst filling said second annular space; means closing off the lower end of said third annular space; means for supplying liquid hydrocarbon charge to the lower section of said vessel and means for removing gaseous products from the upper section of said vessel.

2. The apparatus of claim 1 further limited to the percentage of the area of said first perforated member occupied by perforations progressively increasing at progressively higher levels. i D

3. A process for the hydrocracking of high bolling liquid hydrocarbons to lower boiling gaseous products, which comprises: maintaining a fixed, compactreactton bed of solid hydrocracking catalyst within a confined conversion zone; passing a mixture of recycle hydrocarbons, fresh hydrocarbon charge and hydrogen through said bed in said conversion zone to elfect the conversion to gaseous products; maintaining a confined cooling zone above said reaction zone; flowing the hydrocarbonhydrogen mixture upwardly as a laterally confined column from said conversion zone into said cooling zone;

cooling said mixture in said cooling zone to a tempera-' ture level sufiicient to condense hydrocarbons which it is desired to recycle to said conversion zone but not substantlally below said temperature level; removing hydrogen and gaseous material from the upper section of said cooling zone; accumulating a body of liquid recycle hydrocarbons in the lower portion of said cooling zone and passing liquid recycle from said body downwardly as a laterally confined column into said conversion zone; passing cool, fresh hydrocarbon charge into said cooling zone in indirect heat exchange relationship with the material from said conversion zone to supply the aboveindicated cooling of said material and thereby heat said charge; supplying additional heat to said charge to raise its temperature to a temperature such that when mixed with hydrogen and recycle hydrocarbons, the mixture will be at the desired conversion zone inlet temperature; mixing the heated fresh charge, recycle and hydrogen immediately prior to the entry point to said conversion zone; converting suflicient liquid hydrocarbons to gaseous material in the conversion zone at all times, that the density of the hydrogen-hydrocarbon mixture flowing upwardly is sufficiently below the density of the recycle hydrocarbons flowing downwardly that the hydrostatic head at the lower end of the column of upwardly flowing hydrogen-hydrocarbon mixture is from 3 to psi. less than the hydrostatic head at the lower end of said column of recycle hydrocarbons, sufiicient to force said recycle hydrocarbons into and through said reaction zone and the products of said hydrocracking into said cooling zone by hydrostatic pressure; and controlling the rate at which recycle flows into said conversion zone by controlling the rate of conversion to gaseous products and thereby controlling the density of the reactor eflluent.

4. An apparatus for the catalytic conversion of liquid hydrocarbons to gaseous products, which comprises in combination: a spherically-shaped enclosed reaction vessel; a first perforated partition extending horizontally across the lower section of said vessel and a second perforated partition extending horizontally across the upper section of said vessel; a bed of solid cracking catalyst between said partitions; an enclosed heat exchanger at a level above said reaction vessel; a first conduit extending upwardly from said reaction vessel into the upper section of said heat exchanger; a second con duit extending downwardly from the lower section of said heat exchanger into said reaction vessel at a point opposite the point at which said first conduit enters said vessel; 'and a conduit for the supply of hydrocarbon charge extending into said second conduit adjacent the point of entry of said second conduit to said reaction vessel.

5. An apparatus for the hydrocracking of high boiling hydrocarbons to lower boiling gaseous products, which comprises in combination: an enclosed spherically-shaped reaction vessel; a first foraminous partition extending horizontally across the lower section of said vessel to define a first fluid plenum space therebelow; a second foraminous partition extending horizontally across the upper section of said vessel to define a second fluid plenum space thereabove; a reaction bed of solid, partrculate hydrocracking catalyst in the space between said partitions; an enclosed, elongated heat exchanger maintained at a level above said reaction vessel and inclined at an angle with the horizontal so that one end is lower than the other; a first passageway extending from one of said plenum spaces upwardly into the higher end of said heat exchanger; a vertically extendng weir in the lower section of said heat exchanger; an outlet conduit extending from said heat exchanger and communicating with the space on the side of said weir opposite the side facing said first passageway; means for cooling material in said heat exchanger; a second passageway extending from said heat exchanger adjacent the side of said weir facing said first passageway and between said weir and said first passageway, said second passageway extend ng into said-reaction chamber and into the plenum space which does not communicate with said first passageway; a hydrocarbon supply, conduit extending into said second passageway adjacent its entry to said reaction chamber.

References Cited in the file of this patent UNITED STATES PATENTS 1,934,023 Wright Nov. 7, 1933 2,330,116 Evans Sept. 21, 1943 2,378,728 Roach June 19, 1945 FOREIGN PATENTS 182,556 Germany Feb. 4, 1905 330,0 5 Great Britain June 5, 1930 843,842 Germany July 14, 1952 887,339 Germany Aug.' 24, 1953 

3. A PROCESS FOR THE HYDROCRACKING OF HIGH BOILING LIQUID HYDROCARBONS TO LOWER BOILING GASEOUS PRODUCTS, WHICH COMPRISES: MAINTAINING A FIXED, COMPACT REACTION BED OF SOLID HYDROCRACKING CATALYST WITHIN A CONFINED CONVERSION ZONE, PASSISNG A MIXTURE OF RECYCLE HYDROCARBONS, FRESH HYDROCARBON CHARGE AND HYDROGEN THROUGH SAID BED IN SAID CONVERSION ZONE TO EFFECT THE CONVERSION TO GASEOUS PRODUCTS, MAINTAINING A CONFINED COOLING ZONE ABOVE SAID REACTION ZONE, FLOWING THE HYDROCARBONHYDROGEN MIXTURE UPWARDLY AS A LATERALLY CONFINED COLUMN FROM SAID CONVERSION ZONE INTO SAID COOLING ZONE, COOLING SAID MIXTURE IN SAID COOLING ZONE TTO A TEMPERATURE LEVEL SUFFICIENT TO CONDENSE HYDROCARBONS WHICH IT IS DESIRED TO RECYCLE TO SAID CONVERSTION ZONE BUT NOT SUBSTANTIALLY BELOW SAID TEMPERATURE LEVEL, REMOVING HYDROGEN AND GASEOUS MATERIAL FROM THE UPPER SECTION OF SAID COOLING ZONE, ACCUMULATING A BOYD OF LIQUID RECYCLE HYDROCARBONS IN THE LOWER PORTION OF SAID COOLING ZONE AND PASSING LIQUID RECYCLE FROM SAID BODY DOWNWARDLY AS A LATERALLY CONFINED COLUMN INTO SAID CONVERSION ZONE, PASSING COOL, FRESH HYDROCARBON CHARGE INTO SAID COLLING ZONE IN INDIRECT HEAT EXCHANGE RELATIONSHIP WITH THE MATERIAL FROM SAID CONVERSION ZONE TO SUPPLY THE ABOVEINDICATED COOLING OF SAID MATERIAL AND THEREBY HEAT SAID CHARGE, SUPPLYING ADDITIONAL HEAT TO SAID CHARGE TO RAISE ITS TEMPERATURE TO A TEMPERATURE SUCH THAT WHEN MIXED WITH HYDROGEN AND RECYCLE HYDOCARBONS, THE MIXTURE WILL BE AT THE DESIRED CONVERSION ZONE INLET TEMPERATURE, MIXING THE HEATED FRESH CHARGE, RECYCLE AND HYDORGEN IMMEDIATELY PRIOR TO THE ENTRY POINT TO SAID CONVERSION ZONE, CONVERTISNG SUFFICIENT LIQUID HYDROCARBONS TO GASEOUS MATERIAL IN THE CONVERSION ZONE AT ALL TIMES, THAT THE DENSITY OF THE HYDROGEN-HYDROCARBN MIXTURE FLOWING UPWARDLY IS SUFFICIENTLY BELOW THE DENSITY OF THE RECYCLE HYDROCARBONS FLOWING DOWNWARDLY THAT THE HYDROSTATIC HEAT AT THE LOWER END OF THE COLUMN OF UPWARDLY FLOWING HYDROGEN-HYDROCARBON MIXTURE IF FROM 3 TO 15 P.S.I. LESS THAN THE HYDROSTATIC HEAD AT THE LOWER END OF SAID COLUMN OF RECYCLE HYDROCARBONS,S SUFFICIENT TO FORCE SAID RECYCLE HYDROCARBONS INTO AND THROUGH SAID REACTION ZONE AND THE PRODUCTS OF SAID HYDROCRACKING INTO SAID COOLING ZONE BY HYDROSTATIC PRESSURE, AND CONTROLLING THE RATE AT WHICH RECYCLE FLOWS INTO SAID CONVER SION ZOSNE BY CONTROLLING THE RATE OF CONVERSION TO GASEOUS PRODUCTS AND THEREBY CONTROLLING THE DENSITY OF THE REACTOR EFFLUENT. 